(1) Field of the Invention
The invention relates to an electromagnetic clutch, and in particular, a method of producing a rotor for the clutch and a rotor produced by the method.
(2) Description of the Related Art
In FIG. 12, a prior art electromagnetic clutch 100 is illustrated. The electromagnetic clutch 100 is provided with an electromagnetic coil 101 in the form or a ring, a rotor 102 in the form of a double ring which encloses the electromagnetic coil 101, and an armature 103 which can axially move toward the rotor 102 when the coil 101 is energized. The rotor 102 includes inner and outer walls 107 and 108 and an end wall 105 connected to the inner and outer walls 107 and 108. The outer surface of the end wall 105 defines a friction surface with which the armature 103 engages when the coil 101 is energized.
The armature 102 includes slots 104, which are equally spaced along a circle, for magnetic shielding while the end wall 105 of the rotor 102 includes slots 106a and 106b for magnetic shielding. The slots 106a and 106b are equally spaced along two concentric circles in the end wall 105 of the rotor 102. The magnetic path is deformed as indicated by dotted line .alpha.1 by the slots 104, 106a and 106b to increase the magnetic attractive force by the coil 101.
The rotor 102 is formed by a one-piece body by cold forging, and the end wall 105 is slotted by press working to provide the slots 106a and 106b.
With reference to FIG. 13, there is shown another type of rotor 112 which comprises inner and outer walls 112 and 117 and an end wall 119. The end wall 119 is separately formed into a ring and connected to the inner and outer walls 117 and 118 by filling a non-magnetic material such as copper between the inner and outer walls 117 and 118 and the end wall 119 to provide magnetic-shielding portions 116a and 116b. Such a rotor is disclosed in, for example, Japanese Examined Patent Publication (Kokoku) No. 57-43768.
The above prior art rotors have the problems stated hereinafter. The rotor 102 of FIG. 12 has circumferential bridge portions between the slots 106a and between the slots 106b. The bridge portions allow the magnetic leakage which reduces the magnetic force between the rotor 102 and armature 103.
Furthermore, in the rotor 102, the area of the magnetic pole between the slots 106a and 106b is limited since the slots 106a and 106b and parallel to each other. The limited area of the magnetic pole limits the magnetic flux between the slots 106a and 106b and the magnetic attractive force between the rotor 102 and the armature 103, which limits the maximum transmitted force of the electromagnetic clutch 100. To solve this problem, it can be arranged that the slots are arranged in the end wall so as to diverge from each other tward the outside of the rotor by laser machining or other trimming machining method. However, such a machining increases the time and cost for production.
The rotor 112 of FIG. 13 does not include the bridge portions since the magnetic-shielding portions 116a and 116b are formed in concentric circles. However, the rotor 112 comprises the separate inner, outer and end walls 117, 118 and 119 which are connected by the magnetic-shielding portions 116a and 116b, which increases the number of the elements. Further, during the producing the rotor 112, it is difficult to precisely realize the alignment of the elements since. Thus, the production cost is increases.